1. Field of the Invention
This invention concerns laser patterning or laser drawing technology used in various kinds of micromachining processes for semiconductor manufacture, hologram manufacture and other activities. This invention also concerns a hologram master for the reproduction of color three-dimensional images and technology for manufacture of same.
2. Description of the Related Art
FIG. 23 shows a drawing of the principle of a conventional Lippmann-type hologram. A Lippmann-type hologram is also called a volume hologram, and is recording material with interference fringes recorded on the inside in the thickness direction. A Lippmann-type hologram can utilize its superior color selectivity and angular selectivity to control the phase of light passing through the hologram interior, to control transmission and reflection of specific colors of light, and to cause transmitted light to converge in an arbitrary direction or to control reflection of light incident from a specific direction, apart from recording of three-dimensional images. It can also be used as an optical element having the functions of filters and lenses.
As shown in the drawing, the apple 81, as the object, is placed at an appropriate distance from the photographic plate 60. After passing through the shutter 35, the beam emitted from the gas laser 36 is condensed by the objective lens 33, and is rendered a parallel beam by the collimating lens 31. Thereafter it is divided by the half-mirror 66, with part irradiating the apple 81, and the reflected light arriving at the photographic plate 60 as a signal wave 63. The other part is reflected by the mirror 61 and arrives at the photographic plate 60 as a reference wave 64.
The thickness of the recording material layer of the photographic plate 60 is several tens of times the wavelength of the beam emitted by the gas laser 36. In the case of the optical system shown in the figure, the reference wave 64 and signal wave 63 are incident from opposite directions, so that interference fringes are nearly parallel to the surface of the photographic plate 60, diffraction efficiency is high, and a hologram with prominent wavelength selectivity is obtained.
Generally, in order to use optical lithography to manufacture the master or similar to obtain relief-type holograms, a photomask fabrication process, exposure process, development process, and etching process are necessary. In the photomask fabrication process, an XY table-type laser patterning device, shown in FIG. 22, is used. In this figure, the processed member 1104 is photoresist which is the object of laser patterning, applied to a prescribed thickness on glass substrate.
In an XY table-type laser patterning device, two orthogonal sliders 1102, 1103 are driven to move the processed member 1104 placed on the XY table 1101 in the X-direction and Y-direction, while causing laser light 112 to be transmitted through the electrooptical modulator 103 and acoustooptical modulator 102, reflected by the mirror 104, condensed by the objective lens 105 to form a laser spot on the processed member 1104, to draw the prescribed pattern in order to form the photomask.
Normally, each pixel of the photomask assumes one of two levels, either transparent or opaque. If the depth of the hologram is assumed to be eight levels, then because 8=23, a combination of at least three photomasks is necessary. In the exposure process, a photomask obtained by the above photomask fabrication process is placed in close contact with quartz substrate coated with photoresist, for exposure from above. When there are three photomasks, exposure is performed three times, changing the quantity of light from the light source.
Thereafter development is performed, and the height of each pixel deepens according to the amount of light in the three exposures. Next the photoresist is used as a mask to etch the quartz substrate, transferring the pattern of the photoresist to the quartz substrate, to obtain a hologram master.
However, in the above photomask fabrication process, when a pattern is drawn using an XY table-type laser patterning device, if there is a large number of pixels the number of slider motions in the XY directions and the number of accelerations and decelerations increase, with the inexpedient result that drawing time is increased.
Further, when the interior of a pattern is filled in there are numerous repetitive motions, and during high-speed operation a considerable load is applied to the linear motor, while at the same time the reactive force during acceleration and deceleration of the X-Y table itself becomes a source of vibrations, so that positional precision and velocity precision are decreased.
In Japanese Patent Application Laid-Open No. S59-171119 is disclosed technology for high-speed patterning through disc rotation and the linear motion of an optical system; but there is no disclosure of a method to improve the drawing position precision or the resolution of the drawn pattern.
In Japanese Patent Application Laid-Open No. H10-11814 is disclosed a method for formation of a display pattern to display desired characters or similar, by converting the original-image data expressed in an X-Y coordinate system into an R-θ coordinate system. But there is no disclosure of technology for the highly precise drawing in a short length of time of a pattern having minute gray scales in the depth direction.
Further, the fabrication of a plurality of photomasks is necessary, and in the exposure process also alignment and exposure steps in a number equal to the number of photomasks are required, so that manufacturing time and cost are both increased considerably.
In the Lippmann-type hologram shown in FIG. 23, the signal wave 63 and reference wave 64 must interfere, and so ambient light, fluorescent light, and other general lighting are not suitable as the light source. Hence the object must be placed in a darkroom in which ambient light is shut out, so that buildings in ambient lighting are not suitable as objects. And, because the laser light must irradiate the entire object, large articles are not suitable as objects.
In the article by T. Yatagai et al. appearing in Appl. Opt., 28, 1042–1043 (7989) is disclosed binary-level CGH fabrication technology which employs a control system combining the rotation of a turntable and the linear motion of a slider; but there is no disclosure of multilevel CGH fabrication technology having a plurality of phase values.